It is desirable to actively guide certain ballistic bodies over the earth's surface. By way of example, a gun-fired artillery shell follows a ballistic trajectory and is sometimes accompanied by a complex guidance, navigation and control system that attaches, at least in part, to the shell's body to guide the shell to the desired target. Such guidance systems require the local vertical direction of the body along and during the ballistic trajectory. The local vertical direction generally corresponds to the angle between the body's velocity vector and the plane containing the velocity vector and the earth's gravitational vector.
In the prior art, such local vertical systems typically include either optical sensors or accelerometers. One prior art optical sensor system, for example, has a plurality of optical sensors mounted at intervals around the circumference of a spinning ballistic projectile, such as an artillery shell. A series of ground-based lasers are set at selected points near the trajectory's ground track to operate in conjunction with the on-board optical sensors. These lasers generate beams which follow the shell in flight and--as the shell's ballistic body spins about its body spin axis--the on-board optical sensors detect the beams. The detected laser beam information is then used in conjunction with the known positions of the ground-based lasers to derive the local vertical direction of the shell in body coordinates.
There are several disadvantages of the prior art optical sensor local vertical system. First, the system is not autonomous because it operates only in conjunction with on-ground instrumentation, e.g., the lasers. Second, the on-ground laser network must be trained and controlled on the body during flight, which requires complex electro-mechanical and sighting assemblies. Further, the on-ground instrumentation is typically expensive and is not always available, e.g., in the middle of enemy territory.
One prior art accelerometer local vertical system, for example, has two accelerometers with mutually orthogonal input axes, each of which is orthogonal to the body spin axis. These accelerometers are used to measure the lateral accelerations induced in a spinning artillery shell which flies at a lateral angle-of-attack, known as the "yaw of repose." These lateral accelerations, including their amplitudes and directions in local coordinates, e.g., down, East and North, are then predicted based upon apriori knowledge of the aerodynamic coefficients and the ballistic trajectory. The lateral accelerations can then be used to calculate the local vertical direction in body coordinates.
There are several disadvantages of the prior art accelerometer-based local vertical system. First, aerodynamic effects such as wind gusts introduce perturbations in the lateral accelerations which corrupt the estimate of the local vertical direction. Second, the lateral accelerations induced by the "yaw of repose" are often of small amplitude, and therefore yield relatively small signal-to-noise ratios, reducing the accuracy to which local vertical direction can be determined.
It is, accordingly, an object of the invention to provide apparatus for autonomously determining the local vertical direction of ballistic bodies along a trajectory without the use of accelerometers or optical sensors.
Another object of the invention is to provide a local vertical system which reduces and/or removes the problems described above in connection with the prior art.
Yet another object of the invention is to provide methodology for determining a local vertical direction of a ballistic body without required on-ground assistance and without required use of accelerometers.
Still another object of the invention is to provide apparatus and methods which provides the local vertical direction of a spinning or non-spinning projectile and with increased accuracy.
These and other objects will become apparent in the description which follows.